1. Field of the Invention
The present invention relates to an engine system equipped with an internal combustion engine as a drive source, and in particular, to such an engine system equipped with a thermal storage device that supplies the heat stored in itself to the internal combustion engine.
2. Description of the Related Art
There has hitherto been known an engine system having an internal combustion engine as a drive source, and a thermal storage device (thermal storage tank) that serves to accumulate or store cooling water heated by the internal combustion engine during operation thereof in a heat retaining state so as to utilize it for facilitating the subsequent warm-up of the internal combustion engine. In an engine system described in a first patent document (Japanese patent application laid-open No. 2002-122061) for example, the cooling water (hot water) accumulated or stored in a thermal storage tank is supplied to the internal combustion engine when the temperature of the cooling water falls below a predetermined value during engine starting, so that the driveability and the exhaust characteristics of the internal combustion engine at the time of engine starting can be improved by such supply of hot water.
However, the hot water supplied from the thermal storage tank flows into a cooling system of the internal combustion engine through a prescribed passage or path, thus heating intake port inner walls of a cylinder head. Here, though the temperature of the internal combustion engine is gradually raised by the hot water being supplied to the cooling system, the mode or manner of the temperature rise varies depending upon the various parts of the internal combustion engine.
For example, in the cylinder head of an in-line four-cylinder internal combustion engine, when hot water is made to flow into a water jacket in the cylinder head from one end of a cylinder row to the other end thereof, the rise speed of the temperature is the fastest at a cylinder (intake port) closest to a hot water incoming location and is the slowest at a cylinder (intake port) farthest to that location. In other words, there arises a variation in the temperatures of a plurality of intake ports as a result of the hot water supply. This temperature variation, resulting from a difference between the temperature rise speeds of the respective intake ports, becomes greater in accordance with the increasing operation time (duration time) of the hot water supply.
Such a temperature variation generated between the plurality of intake ports invites a variation in the combustion states between the plurality of cylinders, thus giving rise to a fear that the driveability and exhaust characteristics of the entire engine system might be deteriorated.